da.4: Add some sysctl descriptions

Add descriptions for the following sysctl's:

• kern.cam.da.default_softtimeout
• kern.cam.da.disable_wp_protection
• kern.cam.da.enable_biospeedup
• kern.cam.da.enable_uma_ccbs
• kern.cam.da.poll_period
• kern.cam.da.send_ordered

While here, fix some mandocs warnings.

PR: 258727
Reported by: Felix Johnson <felix dot the dot red at gmail dot com>
Reviewed by: debdrup, pauamma, gbe
Differential Revision: https://reviews.freebsd.org/D32278

(cherry picked from commit 53af9c235f5d368ce788d99e16951a167a152bbc)